1. Field of the Invention
This invention relates to thermal detectors. More particularly, it relates to thermal radiation detector useful for the detection of infrared rays and temperature, such as pyroelectric infrared sensors, thermopiles or thermistor-bolometers, and to a process for manufacturing the same.
2. Description of the Prior Art
Pyroelectric elements, which can detect changes of surface charges caused by the pyroelectric effect resulted from the temperature rise by the thermal energy of incident infrared rays, have been used as thermal detectors, such as infrared and temperature sensors. Such pyroelectric elements consist of an pyroelectric material and a pair of smooth-surface, membrane electrodes formed on both faces thereof, in which one of the membrane electrodes is used as heat-absorbing face. As the pyroelectric material, is used a ceramic material such as a compound of Sr.sub.1-x Ba.sub.x Nb.sub.2 O.sub.6, Pb.sub.5 Ge.sub.3 O.sub.11, TGS (triglycine sulfate) and its analogues, PbTiO.sub.3, PbTi.sub.x Zr.sub.1-x O.sub.3 or LiTaO.sub.3, or an organic compound such as PVDF (polyvinylidene fluoride). Various attempts have been made in order to enhance detection sensitivity: use of a metal layer of higher heat absorption (e.g., Cr and Ni-Cr deposited layers), as the heat-absorbing membrane electrode, in place of the conventional Pt and Au deposited layers; and application of a carbon paste layer by coating, or of a gold black layer by vacuum deposition, on the surface of the conventional Pt or Au deposited layer [W. R. Blevin et al. "Influence of Black Coating on Pyroelectric Detectors", Applied Optics, 13, No. 5, 1171-1178 (1984)].
These techniques, however, have disadvantages of their own. The use of the Cr or Ni-Cr layer is insufficient in heat absorption efficiency because it still has some reflectivity. The layer of gold black deposited on the surface of a membrane electrode is frequently insufficient in heat absorption efficiency and is low in sensitivity particularly in long wavelength regions (10 to 25 .mu.m). The coated layer of carbon paste tends to be thick, and it is difficult to achieve a thickness less than 1000 .mu.m. This means an increased thermal capacitance of the whole thermal detector.
In addition, the problem common to all of these is the difficulty in forming a layer exactly on the surface of membrane electrode; metal or carbon paste tends to be deposited or coated on areas outside the electrode surface, causing unevenness in sensitivity.
This invention was accomplished under such circumstances to provide thermal detectors excellent in heat absorption efficiency and in detection sensitivity for infrared rays.